The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A method as well as an apparatus for the setting of the torque transmitted by a friction clutch are known from WO 03/025422 A1 of the same applicant. The content of this reference, in particular the structure of the friction clutch as well as of the control transmission for the friction clutch described therein, and also the method described in this known application for the setting of the transmitted torque, the calibration method described for the compensation of wear within the friction clutch and the method for the setting and determination of the respective angle of rotation of the electric motor are incorporated by reference in its entirety in the disclosure content of the present application.
A method and an apparatus of the initially named kind are used, for example, in transfer cases by which the torque to be transmitted to the drive wheels of a vehicle is set variably in dependence on the respective driving situation. In this setting procedure, not only a controlled engagement of the friction clutch is required, but also in each case a longer operation with a precisely set torque in accordance with the torque to be transmitted. Depending on the dynamic driving demands, which may, for example, be dependent on the manner of driving of the vehicle operator or on environmental influences (such as a slick road surface with slip occurring), a more frequent or less frequent variation takes place in the amount of the torque to be transmitted, which is reflected in a correspondingly different number of setting procedures of the electric motor.
In accordance with WO 03/025422 A1, the angle of rotation of the electric motor or of a link of the actuator chain may be set to a value corresponding to the torque to be transmitted for the setting of the desired torque to be transmitted. For this purpose, a torque/angle dependence is determined empirically which may, for example, be stored as a characteristic in the form of a table or of a function and which is used for the determination of the angle of rotation corresponding to the desired torque.
It is, however, problematic here that due to wear within the friction clutch and in particular due to disk wear, a displacement of the characteristic may occur that has to be taken into account in the setting of the torque to be transmitted. In accordance with WO 03/025422 A1, the characteristic may therefore be recalibrated at specific time intervals, which is called a calibration procedure or also a post-calibration within the framework of the present application. Within the framework of this post-calibration, the electric motor is, for example, as a rule controlled in the direction “close clutch” on the shutting down of the vehicle motor until the disks of the friction clutch come into contact with one another, which results in a rise in the motor current. A first correction angle for the torque/angle dependence may be determined from the measured current value and the then currently determined angle of rotation, as is described in detail in WO 03/025422 A1. In every calibration procedure, this first correction angle may be updated and stored in order to be able to achieve a setting of the desired torque by a corresponding setting of the angle of rotation based on the original torque/angle dependence while additionally using the first correction angle. It is generally also possible that the then current torque/angle dependency is in each case varied and stored in accordance with the determined first correction angle so that the respective then currently stored torque/angle dependence may be used directly for the setting of the torque to be transmitted.
It is problematic with this post-calibration that it is not the actually transmitted torque, but rather the power consumption of the electric motor, which is determined via a given torque/power consumption dependence, which is measured for the determination of the first correction angle (i.e., for the determination of the displacement of the characteristic used for the torque/angle dependence). This in turn, analogously to the torque/angle dependence, corresponds to a characteristic that may, for example, be stored as a table or as a function. The torque/power consumption dependence may in this connection likewise be determined in a base calibration with reference to corresponding measured values.
The efficiency of the actuator transmission may change over time due to wear within the actuator chain, for example at the reduction gear unit, which is preferably designed as a worm gear or as a helical gear, so that the values for the torque/power consumption dependence determined in the base calibration have to be corrected. In this connection, either an increase in efficiency or a decrease in efficiency may occur in dependence on the respective design of the reduction gear unit. For example, with a metal-to-metal pairing of a worm gear, the surfaces of the intermeshing teeth are worn down over the course of time so that their surface roughness decreases, whereby an increase in efficiency occurs. With a plastic-to-metal pairing, wherein plastic with embedded carbon fibers is used, the carbon fibers that come to the fore due to wear can, in contrast, provide an increased friction and thus a reduction in efficiency.
Both changes in efficiency have an effect on the power consumption of the electric motor during the post-calibration and thus also influence the angle of rotation used as the basis for this calibration. The wear within the actuator chain thus falsifies the result determined by the post-calibration so that ultimately the torque set in operation via the position of the angle of rotation does not exactly correspond to the desired torque to be transmitted.
Attempts have therefore already been made to take account of the wear occurring within the actuator chain and the thereby caused variation of the torque/power consumption dependence, in addition to the post-calibration, in that the respective mileage of a motor vehicle provided with the friction clutch is determined and a correction of the torque/power consumption dependence is carried out in dependence on the mileage or a determined first correction angle is subsequently corrected.
It is disadvantageous with such a kilometer-dependent correction that different driving behaviors of the motor vehicle operator as well as different environmental influences are not taken into account. For example, an aggressive driving behavior means a higher number of setting procedures and thus a greater wear within the reduction gear unit than for example a reticent driving manner. This different wear due to a different driving behavior or due to different environmental influences is not taken into account by a kilometer-dependent correction of the torque/power consumption dependence. Something similar applies when, for example, the kilometers driven are recorded on highway trips, since in this case the wear within the reduction gear unit is relatively small, which is likewise not correctly detected by a kilometer-dependent correction.
It is furthermore not ensured that the respective mileage may in all cases be transferred to a corresponding correction unit via a corresponding interface since such interfaces are not available in all cases. In this case, a correction based on the mileage is completely precluded.